The twin high-rises, named after their luxurious addresses, are legends in the lineage of Chicago high-rises and of modern architecture itself. They are among the first tall embodiments of architect Mies van der Rohe's lifelong meditation on structural clarity. The reticent architect was precise and refined in this work, eloquent from the careful placement of the slabs on the trapezoidal site to the subtly cadenced A-B-B-A rhythm of windows along the perimeter.

The most radical feature of the building, though now trite to contemporary eyes because of the countless knock-offs, is the skin-and-bone expression of the steel and glass towers. The steel I-beams effortlessly define the structure while the glass suspends and encloses space. It was, in a way, two buildings that so simply assert their essence that they dematerialize and become 'almost nothing'.

The popular criticism about these buildings has to do with Chicago's building code and its mandate to fireproof steel buildings more than one story in height. Mies had to cover and fireproof his steel structure. He subsequently added I-beams of different scales onto the facade for the columns and the mullions. This detail was derided because critics claimed that the appliqués presented a falsehood, a facade not in harmony with Mies' doctrinarian belief about structure and truth.

In response, Mies replied:

"... first I am going to tell you the real reason for those mullions, and then I am going to tell you a good reason by itself. It was very important to preserve and extend the rhythm which the mullions set up on the rest of the building. We looked at it on the model without the steel section [I-beams] attached to the corner columns and it did not look right. That is the real reason. Now the other reason is that the steel section was needed to stiffen the plate which covers the corner column so this plate would not ripple, and also we needed it for strength when the sections were hoisted into place. Now, of course, that's a very good reason-but the other one is the real reason."